Modulation调制.pdf

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1、ModulationEvanEverettandMichaelWuELEC433-Spring2013QuestionsfromLab1?Modulationx(t)=Asin(ωt+φ)CarrierData10100Modulation•Goal:overlaydataontocarriersignal(sinusoid)•Sinusoidshavetwoveryaccessibleparameters•ModulateamplitudeandphaseModulation1)InterferenceavoidanceW

2、hynot?2)Highfreq→smallantennasData10100Modulation•Goal:overlaydataontocarriersignal(sinusoid)•Sinusoidshavetwoveryaccessibleparameters•ModulateamplitudeandphaseSignalRepresentation:Phasor•Polar:Amplitude&Phase•Rectangular:“In-phase”(I)&“Quadrature”(Q)π/2QAmplitudeI

3、m[x]Phaseπ0IRe[x]-π/2x(t)=Asin(ωt+φ)x(t)=Icos(ωt)+Qsin(ωt)I=Asin(φ)Q=Acos(φ)SignalRepresentation•Rectangular(I,Q)formsuggestsapracticalimplementationIQIm[x]Icos(ωt)Re[x]10100Icos(ωt)+Qsin(ωt)90˚sin(ωt)Q•Modulation=mappingdatabitsto(I,Q)valuesDigitalModulation[01][1

4、0][00][11]•Mapsbitstocomplexvalues(I/Q)(focusoftheLab3)•Complexmodulatedvaluesarecalled“symbols”•Setofsymbolsiscalled“constellation”or“alphabet”•#ofsymbolsinconstellationis“modulationorder”,M•M-orderconstellationcanencode______bitspersymbolDigitalModulation[01][10]

5、[00][11]•Mapsbitstocomplexvalues(I/Q)(focusoftheLab2)•Complexmodulatedvaluesarecalled“symbols”•Setofsymbolsiscalled“constellation”or“alphabet”•#ofsymbolsinconstellationis“modulationorder”,M•M-orderconstellationcanencodelog2(M)bitspersymbolPhaseShiftKeying(PSK)•Enco

6、desinformationonlyinphaseBPSK(M=2)QPSK(M=4)8-PSK(M=8)[000][001][00][01][0][1][10][11]•Constantpowerenvelope•Pros:noneedtorecoveramplitude,noneedforlinearamplifier•Con:wastesamplitudedimensionQuadratureAmplitudeModulation(QAM)•Encodesinformationinbothamplitudeandphas

7、e√√•(I,Q)grid∈M×M4-QAM16-QAM64-QAM•Commoninwidebandsystems:802.11b802.11g/n802.11ac16-QAM64-QAM256-QAMBit-to-SymbolMapping•Confusingwithneighborismostlikelyerror•Besttominimizebit-differencebetweenneighbors•GrayCoding•Neighboringsymbolsdifferbyonlyonebit•Extraperfo

8、rmanceatzerocost(thisisrare!)Natural-coded[01][10]Gray-coded[01][11]QPSKQPSK[00][11][00][10]Tradeoff:Ratevs.ErrorProbability•Byincreasingmodulati